<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<script src="https://code.jquery.com/jquery-1.12.4.js"></script>
<style>
    .color{
        color: red;
    }
</style>
<body>
    <div class="check">
    <input type="checkbox" value="1" name="check" checked="checked" />
    <input type="checkbox" value="2" name="check"  />
    <input type="checkbox" value="3" name="check" checked="checked" />
    <input type="checkbox" value="4" name="check"   />
    </div>
    <table id="tab">
        <tbody>
            <tr>
                <td>第一行</td><td>第一行</td>
            </tr>
            <tr>
                <td>第二行</td><td>第二行</td>
            </tr>
            <tr>
                <td>第三行</td><td>第三行</td>
            </tr>
            <tr>
                <td>第四行</td><td>第四行</td>
            </tr>
            <tr>
                <td>第五行</td><td>第五行</td>
            </tr>
        </tbody>
    </table>
</body>
<script>
    // 遍历checkbox控件输出选中个数
    console.log($(':checked').length);
    $('.check').change(function () {
        console.log($(':checked').length);
    })
    //设置表格序号为偶数行的文本颜色
    var tr=$('#tab tr');
    var len =tr.length
    for (let i = 0; i < len; i++) {
        if ((i+1)%2==0) {
            tr[i].style.color='red'
        }
    }
    // tr=$('#tab tr:odd').css('color','red') jquery
</script>
</html>